Gerrard will use the Bardic Inspiration on the first attack of his that misses:  4

One of Kei's beams of crackling black energy seems to connect, as does his spectral tentacle. But both seem to do only minor damage to the massive monstosity.

Kellen's spectral fist, however, just glances off.

Gerrard closes in further, carving deeply into its tough hide. It definitely felt that. (With an AC of 16, five of those hit, now six with bardic inspiration.)

The aboleths attack, with their own tentacles:

#1 Tentacle Attack #1: 26. Hit: 11 bludgeoning damage.
#1 Tentacle Attack #2: 16. Hit: 12 bludgeoning damage.
#1 Tentacle Attack #3: 28. Hit: 13 bludgeoning damage.

#2 Tentacle Attack #1: 21. Hit: 12 bludgeoning damage.
#2 Tentacle Attack #2: 26. Hit: 14 bludgeoning damage.
#2 Tentacle Attack #3: 24. Hit: 14 bludgeoning damage.

BLOOP! The kraken expels an ink cloud in a 40-foot radius. The area is temporarily heavily obscured to all creatures other than the kraken. 

Then a flash of lightning ripples through the water, felt more than seen. 

Lightning Strike vs. Aboleth: DC 18 Dex Save: 2, taking 24 lightning damage on a failed save, or half as much damage on a successful one.

There's a bit of a current here, and the ink fades quickly. When you can see again, you notice that the kraken has moved to the exit in the north wall.

Kei, Kellen, and Gerrard – may act next (in that order). Gerrard can no longer see the kraken in the gloom.

Kei 

The fierceness of Gerrards onslaught insires Kei to up the ante with his own attacks. He quickly judges the distance from himself to the kraken and it to the aboleths. He plucks at the Weave in a way not too familiar to him but succeeds in sending what would normally be a fireball but this was more like a vibrating, rumbling ball of thunder.

The tentacle skitters across the seabed and once again slaps at the young kraken.

Metagamic Transmuted Spell Fireball to Thunder damage

A - Thunderball DC17 DEX  for 37 thunder damage. Centred at a point to hit only the kraken.

BA - Tentacle move and attack (roll in log)

Kellen

Panting a little now, Kellen figures he'd better take care of himself as a wave of pain washes over him. Trying to see the others through the gloom, he'll grit his teeth and focus on a spot between them all. A comforting feeling pulses through all three of them, closing up wounds and soothing the lightning burns suffered earlier.

16 hp healed each

He'll then close his fist and swing it back around, moving his spectral hand back towards the kraken to try for another blow.

Attack: 23 Damage: 28 force damage

Gerrard:

Seeing the effect of the Thunderball and the magical hand heading off in the same direction, Gerrard follows and soon sees the kraken again.  Moving up back into melee range, he takes up a defensive stance again as he is not close enough to attack the foul rune yet.

Dodge

Kraken DC17 Dex Save vs. Thunderball: 24

Kei's ghostly tentacle is swatted away by one of the kraken's mighty tentacles, but Kellen's spectral fist delivers a massive blow.

It ejects another massive blob of ink. Gerrard and Kei are at the edge of it. (This happened just before Gerrard's turn, so Gerrard may change his turn as needed.)

The aboleths retreat, nearly colliding with Gerrard in their blinded attempt to escape the ink.

When the ink lifts, the kraken is nowhere to be seen.

Kei, Kellen, and Gerrard may act next!

(Gerrard will keep his turn—move 30 feet towards the previous location of the kraken then dodge—but he also calls “Where is it?!”, hoping his allies with darkvision have a better shot of finding it.)

Kei 

Kei watches in disbelief as the kraken shrugs of half the energy of his Thunderball. "Damn this thing back to the depths that spawned it!" He snaps vehemently at the now missing creature. "It's nowhere i can see" he answers gerrard as he turns his attention to the aboleths. "Hey! You two! I thought you were meant to be fearsome beings worthy of the fear and respect of many a sailor? All I saw was a tickle on the hide of a true deep sea terror" Kei vents the frustration of his own failures, directing the anger towards the pair of aboleths.

Kellen

It had to have gone through the wall over here Kellen says, moving toward the hole and bringing his giant hand with him, ready to swing it again at the first glimpse of the creature.

Gerrard:

Already near the edge of the opening in the wall, Gerrard maintains his defensive stance and goes down the passage.

(Dodge plus move 30 feet down the hall until he can at least see the beast)

Seeing the Aboleths near the pit, Gerrard thinks at them, “You are near the arcane sigils.  Destroying them may weaken the kraken.”

"Your critique is irrelevant, mortal," replies one aboleth to Kei.

"Yes, the obscene sigils must be erased," replies the other to Gerrard.

The aboleths glide closer to the pit. One slams its tail against the intricate glowing marks that writhe around the top edge and the upper twenty feet of the pit. 
Tail Attack: 22. Hit: 12 bludgeoning damage.

Kei: Make a Wisdom saving throw.

Gerrard and Kellen swim toward the opening in the north wall; Kellen can see that it was a modest foyer, half-built, now a gaping opening into the seabed beyond. There is no sign of the kraken.

Everyone may act.

A few of the sigils go black upon the damage inflicted by the aboleth. But there are many more left to destroy.

Kellen

Shit he mutters under his breath. Where the hell did it go? No telling what'll happen to the Stye if we lose this thing....of course, no telling what will happen to us if we find it. He sighs and shakes his head resignedly. He'll move further into the room, looking around for any sign of it.

perception 12

Gerrard:

(Gerrard had moved to the opening the previous turn, and last turn he moved through the hole another 30 feet along, if that makes a difference on spotting the creature).

Keeping pace with Kellen, Gerrard readies to protect him should the kraken attack (Readied action to bait and switch,  putting himself engaged with the kraken should it attack the two of them.)

Kei

WIS save 12

There is nothing beyond the partially-collapsed foyer except open seabed. 

One aboleth continues to assault the glowing, undulating sigils around the top edge of the pit with its slimy appendages:

Tentacle Attack: 29. Hit: 13 bludgeoning damage.
Tentacle Attack: 27. Hit: 12 bludgeoning damage.
Tentacle Attack: 12. Hit: 13 bludgeoning damage.

More sigils go black. There is no sign of the kraken.

Everyone may act.

Gerrard:

Moving back in with the others, Gerrard keeps a keen watch out for the kraken.  He pivots and watches, listening for a rush of water or hint of contact in the nearby deeps.

Perception:  12